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UNIVERSITY    OF     .UINO.S     LIBRARY    AT    URBANA-CHAMPA.GN 


I^^B    9197B 

JAN  Je  1976 


SEP  I  8 
MAY  3 


UNIVERSITY  OF  ILLINOIS  BULLETIN 

Vol.  VII.  MARCH  21,  1910.  No.  29 

[Entered  February  14,  1902,  at  Urbana,  Illinois,  as  second-class  matter  under 
Act  of  Congress  of  July  16,  1894.] 

BULLETIN  No.   14 
DEPARTAIENT  OF  CERAMICS 

A.   V.   BLEININGER,  Director 

OPALESCENCE  AND  THE  FUNCTION  OF 
BORIC  ACID  IN  THE  GLAZE 


BY 

R.  T.  STULL  AND  B.  S.   RADCLIFFE 

URBANA,  ILL. 


1909-1910 


PUBLISHED  FORTNIGHTLY  BY  THE  UNIVERSITY 


[Rei'Rinted  from  Transactions  of  American  Ceramic  Society,  Vol    XII. 
Paper  read  at  Pittsburgh  Meeting,  February,  1910.] 


OPALESCENCE  AND  THE  FUNCTION  OF  B.O. 
IN  THE  GLAZE. 

BY 

R.  T.  Stull  and  B.  S.  Radcliffe^  Urbana,  111. 

In  the  study  of  whiteware  glazes,  at  the  University 
of  Illinois,  the  class  in  glaze  making  t-onstnieted  a  group 
of  glazes  which  showed  all  stages  from  clear  colorless 
through  opalescence  to  opaque  white. 

The  group  consisted  of  six  series  having  nine  mem- 
liers,  each  covering  the  following  field : 

.3  K„0      \  r  L'.l  to  6.1  SiO, 

.4  CaO     i 

,1  PbO     '  I      0  to  1.0  1J,(), 

The  glazes  were  ap]»lied  to  biscuit  wall  tile,  set  on 
edge  in  flat  tile  saggers  and  burned  to  cone  3.  Examina- 
tion of  the  trials  seemed  to  indicate  that  opalescence  was 
not  a  function  of  the  quantity  of  boric  oxide,  since  glazes 
having  small  molecular  quantities  appeared  as  opalescent 
or  opaque  as  those  having  larger  quantities. 

There  also  seemed  to  be  a  dividing  line  between  the 
clear  and  the  opalescent  glazes.  Although  the  glazes  had 
run  and  beaded  along  the  edge  of  the  tile,  the  thickness  of 
the  glazes  against  the  white  background  of  the  tile  was  not 
sufficient  to  determine  accurately  the  dividing  line  between 
the  clear  and  the  opalescent  fields. 

In  order  to  determine  more  accurately  this  line,  a 
series  of  glazes  was  inserted  between  what  appeared  to  be 
the  clear  and  opalescent  fields,  and  another  series  added  to 
the  left  of  the  group,  thus  covering  the  following  limits: 


.3  K„0     ^  (1.76  to  6.1  SiO. 

.2  Na,0    (    45  AlO  -' 

.4  CaO     I  -^^       -"^  i 

.1  PbO    .'  '        0  to  1.0  B.,0, 


2  OPALESCENCE  AND  FUNCTION  OF  B0O3  IN  GLAZE. 

Each  circle  oii  the  diagram  (Chart  I)  represents  a 
glaze.  In  the  lower  left  hand  corner  is  low  boric  oxide  and 
low  silica.  In  the  lower  right  hand  corner  is  low  boric 
oxide  and  high  silica.  At  the  npper  left  corner  is  high 
boric  oxide  and  low  silica.  In  the  npjier  right  corner  is 
hio'li  boric  oxide  and  hioh  silica. 


T-fx-Ars'=5.    AM      CEK      &OC. 


STULU    tc      RA^CLIFFE. 


CHI\^T-1 


l.OBA 
1.76  SiOi 


.2K2O  \ 

SNaiOr  ^KAin 
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Total-QR,  excluding  B^O^ 


In  the  series  running  horizontally  from  left  to  right, 
boric  oxide  is  constant  and  silica  increases,  while  in  the 
series  running  vertically  from  bottom  to  top  silica  is  con- 
stant and  boric  oxide  increases. 

In  order  to  detect  faint  opalescence  it  is  necessary  to 
examine  thick  laj-ers  of  the  glazes.  For  this  purpose  tiles 
from  stoneware  clay  were  made  having  conical  depressions 
one-half  inch  in  diameter  and  one- fourth  inch  deep  (Chart 
IV).  These  depressions  Avere  filled  with  the  glazes,  the 
wall  tiles  were  dipped  and  the  trials  burned  to  cone  3. 


OPALESCENCE  AND  FUNCTION  OF  BoOg  IN  GLAZE.  3 

Examination  of  tlie  trials  showed  opalescence  strongly 
in  Series  2  running-  vertically.  Glazes  having  low  boric 
oxide  seem  to  show  as  strong  opalescence  as  those  having 
higher  quantities.  The  members  in  vertical  Series  li/.  do 
not  show  the  slightest  trace  of  opalescence.  To  the  right 
of  the  dividing  line,  opalescence  increases,  while  glazes 
in  the  field  to  the  left  are  perfectly  clear. 


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Some  very  interesting  points  witli  respect  to  crazing 
are  observed.  In  the  A  series,  A-Vo  is  a  basic  matt  and 
crazed.  A-9,  which  is  high  in  silica,  is  badly  devitrified 
and  crazed.    A-8  is  slightly  devitrified  and  slightly  crazed. 

In  the  case  of  these  two  extremes  of  crazing  the  addi- 
tion of  boric  oxide  has  had  diametrically  opposite  effects. 
Boric  oxide  added  to  A-i  ^  has  inci-eased  crazing,  the  craz- 
ing increasing  with  increase  in  boric  oxide.  In  A-8  and  0 
the  addition  of  boric  oxide  has  overcome  crazing  and  also 
devitrification. 


4  OPALESCENCE  AND  FUNCTION  OP  B^O,  IN  GLAZE. 

In  his  article^  on  "The  Function  of  Boron  in  the  Glaze 
Formula/'  Professor  Binns  says:  -'It  was  noticed  that 
from  its  position  in  the  periodic  curve  and  from  its  valence, 
boron  might  be  presumed  to  act  like  alumina,  functionino' 
as  a  base  with  strong  acids  and  as  an  acid  with  strong 
bases." 

He  also  points  out,  that  in  a  glaze  having  the  formula 
RO,  .2  AUOo,  l.GSiOo,  whose  oxygen  ratio  is  one  to  two, 
if  .3  BoOo  be  added  it  is  necessary  to  raise  tlie  silica  to  two 
and  one-half  according  to  practice,  and  that  if  BgOo  be 
considered  as  base  the  oxygen  ratio  then  becomes  one  to 
two. 

Professor  Binns  further  states:  "  *  *  *,  the  opinion 
is  further  expressed  that  BoO;,  undoubtedly  is  basic  when 
melted  with  silica  or  silicates,  and  that  it  may  be  used  to 
replace  alumina  partially  or  entirely,  with  the  exception 
of  its  exercise  of  the  property  of  fusibility." 

Dr.  Zimmer  says:^  "While  I  admit  that  all  the  facts 
brougth  out  seem  to  indicate  that  boron  will  act  as  a  base. 
there  are  suridy  conditions  under  which  it  will  act  as  an 
acid." 

If  we  arrange  the  oxides  of  the  elements  in  groups 
according  to  the  periodic  system,  we  find,  as  a  rule,  that 
the  bases  of  low  molecular  weight  produce  best  opacity 
and  bases  of  high  molecular  weight  least  opacity.  With 
respect  to  acids,  we  find  the  opposite,  i.  e.,  acids  of  high 
molecular  weight  produce  best  opacity  and  acids  of  low 
molecular  weight  least  opacity. 

According  to  Seger's  rules  for  crazing,  bases  of  low 
molecular  weight  produce  least  crazing  and  bases  of  high 
molecular  weight  produce  most  crazing.  Whether  the  op- 
posite is  true  of  acids  has  not  been  definitely  shown. 

By  calculating  the  oxygen  ratios  of  the  glazes  includ- 
ing boric  oxide,  lines  of  equal  oxj^gen  ratios  are  obtained 
running  vertically  across  the  diagram,  as  shown  in  Chart  I. 


'  Vol.  X,  Trans.  A.  C.  S.,  p.  159. 
2  Ibid,   p.   168. 


opalf:scence  and  function  op  B2O3  in  glaze.  5 

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rotalOR,    B^O^asBase 


6  OPALESCENCE  AND  FUNCTION  OF  B^Og  IN  GLAZE. 

CoiisideriDg  boric  oxide  as  aii  acid,  lines  of  equal  oxy- 
gen ratios  run  diagonally  downward  from  left  to  right 
(Chart  II).  Calculating  oxygen  ratios  in  which  boric 
oxide  is  assumed  to  be  a  base,  gives  diagonal  lines  running 
upward  from  left  to  right  which  appear  to  radiate  from  a 
common  center,  (Chart  III). 

In  Chart  I  it  is  observed  that  the  dividing  line  be- 
tween the  clear  and  opalescent  glazes  runs  parallel  to  and 
between  the  oxygen  ratio  lines  of  1  to  2  and  1  to  2.2.  Since 
opalescence  is  shown  so  strongly  on  the  total  oxygen  ratio 
line  of  1  to  2.2,  it  is  reasonable  to  suppose  that  the  true 
dividing  line  between  clear  and  opalescent  glazes  lies  on  or 
very  near  to  the  total  oxygen  ratio  line  of  1  to  2. 

In  consulting  Charts  II  and  III,  it  is  evident  that  the 
lines  of  equal  oxj^gen  ratios  in  which  boric  oxide  is  calcu- 
lated either  as  an  acid  or  a  base,  do  not  bear  any  definite 
relation  to  the  dividing  line  between  the  clear  and  opales- 
cent glazes. 

Boric  oxide  fuses  at  a  low  temj)erature,  forming  a  thin 
water-like  fluid.  In  this  condition  it  is  one  of  the  strongest 
igneous  solvents  at  our  command,  and  there  are  few  inor- 
ganic oxides  that  it  is  not  capable  of  dissolving.  Silica 
added  to  it  is  dissolved  and  evidently  combined,  since 
silica  is  strongly  acid  in  character  in  igneous  fusion,  the 
combination  is  probably  some  silicate  of  boron  which  pro- 
duces a  milky  precipitate. 

Opalescence  in  glasses  containing  boric  oxide  is  not 
caused  by  crj'stallization,  nor  by  that  peculiar  form  of 
segregation  which  sometimes  precedes  it.  If  it  were,  then 
sudden  cooling  would  leave  the  mass  in  an  amorphous  con- 
dition, thus  giving  a  clear  glass.  A  fusion  of  silica  and 
boric  oxide  when  suddenly  cooled  will  show  strong  milki- 
ness.  Fritts  containing  boric  oxide  frequently  show  opal- 
escence though  cooled  suddenly  by  quenching  in  water. 

It  is  trus  that  a  glaze  cooled  slowly  will  show  stronger 
opalescence  than  the  same  glaze  cooled  quickly,  but  it  must 
be  remembered  that  precipitates  require  time  to  form.    As 


OPALESCENCE  AND  FUNCTION  OF  B0O3  IN  GLAZE.  7 

aij   example,   the  comi:)lete  precipitation    of   magnesia   by 
sodium  ammonium  phosphate  recjiiires  several  hours. 

CONSIDERATIONS  ON  CRAZING. 

Boric  oxide  when  added  to  A-Yo  (which  is  basic  in 
character),  undoul)tedl3^  plays  the  pai-t  of  an  acid.  In- 
creasing boric  acid  has  increased  crazing.  The  more  boric 
acid  present,  the  greater  is  the  degree  of  crazing. 

In  the  case  of  A-8  and  9,  which  are  highly  acid  in 
character,  it  is  more  than  probable  that  the  addition  of 
boric  oxide  has  combined  as  a  base,  and  in  so  doing  has 
overcome  both  devitrification  and  crazing. 

In  comparing  the  dividing  line  between  crazing  and 
non-crazing  glazes,  with  lines  of  total  oxygen  ratios  on  the 
three  different  charts,  we  find  that  this  line  does  not  con- 
form to  any  oxj'gen  ratio  lines,  whether  boric  oxide  be  ex- 
cluded from  the  calculations,  or  included  either  as  acid  or 
as  base. 

It  has  been  noted  that  crazing  has  increased  with  in- 
crease in  boric  oxide  in  glazes  whose  previous  total  oxj^gen 
ratios  are  low.  It  has  also  been  stated  that  this  crazing  is 
due  to  the  presence  of  boric  oxide  as  an  acid,  and  as  such, 
whether  free  or  combined,  its  tendency  is  to  reduce  crazing. 
If  boric  oxide  combines  as  a  base,  its  influence  should  be 
that  of  overcoming  crazing,  since  it  is  a  base  of  low  mole- 
cular weight  (in  accordance  with  Seger's  rale). 

If  boric  oxide  when  combined  as  a  base  acts  like  alum- 
ina and  can  replace  alumina,  then,  any  increase  in  boric 
oxide  would  decrease  crazing  the  same  as  alumina,  in  ac- 
cordance with  the  statement  of  Purely  and  Fox.^ 

Boric  oxide  present  as  acid  and  boric  oxide  present  as 
base  influence  crazing  in  opposite  directions,  which  might 
be  expected  since  the  properties  of  bases  and  acids  are 
opposite  in  character.  In  glazes  which  craze  from  excess 
boric  acid,  an  addition  of  silica  has  a  double  influence  in 


»  Fritted  Glazes     Trans.   A.   C.   S.,   Vol.   IX.    p.   ]78. 


8  OPALESCENCE  AND  FUNCTION  OP  B2O3  IN  GLAZE. 

preventiug  crazing.  By  converting  boric  acid  to  boric  base 
it  decreases  crazing  acid  and  increases  non-crazing  base. 

The  reason  tliat  tlie  line  between  crazing  and  non- 
crazing  glazes  does  not  conform  to  anj^  oxygen  ratio  lines 
is  because  both  free  crazing  boric  acid  and  combined  non- 
crazing  boric  base  may  both  be  present,  and  it  takes  a 
definite  amount  of  the  one  to  overcome  the  influence  of  the 
other.  With  reference  to  the  dividing  line  between  devitri- 
fied  and  crazed  glazes  and  non-devitrified  and  non-crazed 
glazes,  Chart  III  shows  that  the  line  coincides  in  direction 
with  the  oxygen  ratio  lines  when  boric  oxide  is  considered 
as  a  base.  But  when  referred  to  Charts  I  and  II,  it  is  shown 
that  the  line  does  not  conform  to  the  oxygen  ratio  lines  in 
either  of  these  two  cases. 

Although  there  are  no  absolute  chemical  proofs  at 
hand,  the  following  theories  are  advanced  from  the  fore- 
going evidence : 

First,  boric  oxide  when  added  to  a  glaze  whose  pre- 
^  ions  oxygen  ratio  is  less  than  1  to  2  is  present  as  an  acid, 
combined,  tending  to  produce  a  chemical  equilibrium.  Any 
excess  is  present  in  acid,  uncombined,  in  solid  solution. 
When  present  in  the  acid  form  its  influence  is  that  of  pro- 
ducing clear  glazes  and  crazing. 

Second,  boric  oxide  when  introduced  into  a  glaze 
whose  previous  oxygen  ratio  is  1  to  2,  is  present  as  an  un- 
combined acid  in  solid  solution,  imparting  clear  and  craz- 
ing tendencies  to  glazes. 

Third,  boric  oxide  when  introduced  into  a  glaze  with 
a  previous  total  oxygen  ratio  greater  than  1  to  2  tends  to 
produce  chemical  equilibrium  by  combining  as  a  base,  thus 
producing  opalescence  and  non-crazing  tendencies.  Any 
excess  boric  oxide  is  present  as  an  uncombined  acid  in  solid 
solution. 

Fourth,  opalescence  is  caused  by  the  precipitation  of  a 
silicate  in  which  boric  oxide  is  evidently  basic.  The  in- 
tensity of  opalescence  does  not  depend  upon  the  quantity 
of  boric  oxide  present  but  upon  the  quantity  combined  as 
a  silicate. 


OPALESCENCE  AND  FUNCTION  OF  BgOg  IN  GLAZE.  9 

It  is  interesting-  to  note  that,  although  the  line  divid- 
ing crazing  and  non-crazing  glazes  does  not  conform  to  any 
(jxygen  ratio  lines,  it  conforms  nearest  to  the  lines  of  equal 
oxygen  ratio  ^^•hen  boric  oxide  is  assumed  to  be  base.  The 
oxygen  ratio  line  of  1  to  2,  when  boric  oxide  is  considered  a 
base,  passes  through  the  middle  of  the  field  of  best  glazes 
found  in  this  group. 

DISCUSSION. 

Mr.  Parinelee:  I  have  this  suggestion  that  I  wish 
some  of  those  interested  in  the  matter  of  opalescence  would 
determine  A\hether  that  is  opalescence  or  fluorescence. 
Opalescence,  as  I  understand  it,  is  due  or  at  least  com 
monly  attri])uted  to  internal  fractures,  while  fluorescence 
is  a  different  thing.  It  would  be  a  comparatively  simple 
thing  to  take  a  sample  of  glaze  showing  this  description  of 
opalescence  and  determine  which  it  is.  I  think  it  Avouhl 
be  a  nmtter  of  some  interest. 

Mr.  ^'^TiiJI:  The  trials  just  came  out  of  tlse  kiln  ;i 
little  while  before  the  convention.  The  intention  was  to 
have  thin  sections  made  of  them  and  examine  them  micro- 
scopically. We  used  the  term  opalescence  because  that  is 
the  term  most  generally  understood,  and  because  it  ap- 
pears like  opalescence, — it  has  the  color.  We  know  that 
some  precipitates  are  opalescent  in  appearance.  The  pre- 
cipitates of  aluminum  hvdroxide  and  silicic  acid  are  sug 
gestively  opalescent.  Kichards,  in  his  work  on  the  determ- 
ination of  the  atomic  weight  of  silver,  jjroduced  strong 
opalescence  in  the  silver  chloride  which  passed  through 
the  filter.  The  presence  of  minute  solid  particles  in  sus- 
pension may  give  the  reflection  of  light  nearly  as  well  as 
cracks  or  flaws  will  do. 

}fr.  ^Shaic :  I  have  had  some  experience  with  borax 
but  I  do  not  believe  I  can  add  anything.  I  have  been  won- 
dering what  is  the  relation  between  opalesc(»nce  and  opac- 
ity. Substances  that  produce  opacity  if  fused  hard  enough 
will  produce  opalescence.     I  have  produced  white  enamel 


10  OPALESCENCE  AND  FUNCTION  OF  B0O3  IN  GLAZE. 

and  very  often  had  it  "burned."  The  opac-ity  is  all  burned 
out  of  it,  and  tlie  glaze  after  that  time  very  often  shows  a 
great  amount  of  opalescence. 

Mr.  Binns:  Would  you  define  opalescence  as  a  par- 
tial opacity? 

Mr.  ^haw  :     That  is  exactly  what  I  think. 

Mr.  ^tull :  I  agree  with  that,  that  opalescence  is  a 
''dilute"  state  of  opacity.  Here  is  F9,  (exhibits  trial),  the 
one  highest  in  BoOg,  and  highest  in  silica.  It  shows  opaque 
white. 

Mr.  Bin  us  :  I  would  like  to  ask  Mr.  Stull  whether  he 
has  any  explanation  of  the  connection  between  devitrifica- 
tion and  crazing.  We  have  generally  considered  the  two 
things  were  influenced  by  opposite  conditions.  I  would  be 
interested  to  know  what  suggestions  he  has  to  make  on 
that  p'oint. 

Mr.  HtuU:  Most  generally  crazing  is  conceded  to  be 
a  function  of  the  co-efficient  of  expansion  and  contraction, 
but  personally  I  think  there  is  another  factor  which  we  do 
not  consider,  and  that  is  crystalline  tension.  We  know 
that  crystals  form  from  gases,  from  fluids  and  from 
solids.  As  a  time-worn  illustration,  the  iron  beams  of  rail- 
road bridges  frequently  become  so  crystalline  from  the  jar 
of  the  trains  that  they  become  weak  and  break.  We  have 
plenty  of  evidence  of  crystallization  going  on  in  solids.  A 
substance  in  crystallizing  usualh^  does  one  of  two  things, 
\\  either  increases  in  volume  or  decreases  in  volume.  In 
either  case  if  crystallization  should  continue  down  into  the 
solid  glaze  it  would  produce  a  tension  strong  enough  to 
cause  crazing.  In  experimenting  with  crystalline  glazes 
of  the  type  which  crystallize  in  assicular  groupings,  I  have 
failed  to  make  one  crystalline  glaze  yet  which  did  not 
craze.  In  these  crystals  which  radiate  from  centers,  it  is 
interesting  to  note  that  the  craze  marks  did  not  run  in 
irregular  zigzag  lines,  but  ran  approximately  in  broken 
concentric  rings  around  the  crystalline  center  in  this  man- 
ner.    (Indicating  on  the  board  as  shown  herewith.)     If  it 


OPALESCENCE  AND  FUNCTION  OF  B0O3  IN  GLAZE.  11 

is  not  due  to  crystalline  tensions,  then  what  is  it  clue  to? 
Crystalline  glazes  with  oxygen  ratios  all  the  way  from  one 
to  one  up  to  one  to  five  Avill  craze  if  they  are  crystallized. 
In  practice  I  tried  to  overcome  it  by  slow  cooling  from 
vanishing  red  heat  down.  The  slower  the  cooling  the  worse 
the  crazing  appeared. 


FlssLCUtar    Grouizin^ 
Showinq  Craze  Mccr/Cs 
ff  vi2Tiinq  Concentrcca//y. 


Mr.  Binns:  I  would  like  in  that  connection  to  make 
another  suggestion.  We  commonly  theorize  or  philosophize 
this  way :  Silica  in  a  body,  being  crystalline,  has  a  high 
€0-efficieiit.  In  the  case  of  excess  of  silica  in  the  glaze  and 
consequent  devitrification,  it  seems  to  me  quite  possible 
that  the  silica  has  returned  to  its  crystalline  form,  and 
thus  regained  its  high  co-efficient.  I  have  no  doubt  that 
Mu.  StulTs  theory  of  crystalline  tension  strains  has  some- 
thing to  do  with  it.  At  the  same  time  I  think  the  fact  that 
the  silica  has  returned  to  its  crystalline  form  is  also  an 
important  factor.  I  have  frequently  noted  in  that  connec- 
tion that  a  short  fired  glaze  almost  invariably  crazes  sim- 
ply because  the  silica  has  not  been  thoroughly  taken  into 
the  solution,  and  has  thus  not  reached  the  amorphous  form 
with  the  low  co-efficient. 

And  now  the  question  of  boron,  in  which  I  am  natur- 
ally very  much  interested.  I  would  like  to  say  just  a  word 
on  that.  This  is  the  first  time  it  has  been  alluded  to  since 
I  brought  it  up  two  years  ago.  It  is  quite  gratifying  to 
find  that  at  least  a  portion  of  my  contention  is  substan- 
tiated. Dr.  Singer  showed,  in  his  paper,  that  it  was  pos- 
sible to  produce  compounds  in  which  boron  sesquloxide,  or 
boric  oxide,  functions  as  a  base. 


12  OPALKSCENCE  AND  FUNCTION  OF  BoOs  IN   GLAZE. 

Ever  since  the  paper  was  presented  two  years  ago  T 
have  worked  upon  the  theory,  which  I  then  enunciated, 
that  in  a  normal  glaze  under  normal  conditions  boric  oxide 
functions  as  alumina.  And  my  students  in  the  blackboard 
work  have  said  to  me,  why  do  you  put  boric  oxide  under 
the  alumina  when  all  the  books  do  the  other  thing  and  put 
it  under  the  silica?  I  tell  them  if  they  live  long  enough 
they  will  find  that  all  the  books  will  have  it  under  alumina. 

Mr.  Stull  has  brought  up  certain  exceptional  condi- 
tions, as  for  instance,  in  a  liighh^  basic  glaze,  where  it  is 
quite  probable  that  a  portion  of  the  boric  acid  or  all  of  it 
may  function  acidically.  And  by  the  Avay,  I  wish  we  had 
some  better  term  than  acid  and  base  to  express  this  action. 

For  many  years  I  groped  in  the  dark  with  regard  to 
fritted  glazes,  and  I  believe  a  good  many  are  doing  so  still. 
But  when  we  once  established  that  the  bi-silicate  ratio  was 
the  best  thing  for  a  clear  glaze  under  proper  conditions, 
there  did  not  seem  to  be  any  philosophy  or  working  reason 
why  fritted  glazes  should  not  come  under  the  same  rule,  in 
other  words,  why  boric  oxide  should  introduce  an  entirely 
new  set  of  laws.  But  by  the  simple  arrangement  of  placing 
boric  oxide  with  the  sesquioxides  the  whole  thing  is  solved. 
Fritted  glazes  now  obey  the  same  law.  They  are  normally 
bi-silicate,  or  closely  approximating  it.  I  have  no  more 
difficulty  now  in  laying  down  fritted  glazes  than  I  used  to 
have  in  raw  glazes.  But  until  this  point  was  reached  there 
was  no  law  which  could  be  used  as  a  guide. 

We  are  very  glad  to  have  this  word  from  Mr.  Stull 
not  only  on  the  boric  question,  but  on  that  rather  obscure 
and  interesting  phenomenon  of  opalescence  wliich  has  oc- 
cupied us  today.  He  has  also  demonstrated  his  work  in  a 
Acry  interesting  manner.  His  enclosing  the  panel  of  glaze 
in  the  cell  is  quite  a  new  idea  and  one  well  worthy  of  con- 
sideration, when  we  come  to  deal  with  the  consistency  of 
the  glaze  matter. 


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